PzCampaign - question on air power usage

[Bostonian]

Does air support rollover if I don't use a specific bomber or fighter in a given turn? I was wondering if I could hoard air support for a few turns in preparation for a big assault. Or is it a situation more akin to: 'if you don't use it, you lose it' for each turn? 
I'm thinking particularly of France '40 though wondering if it is different for other PzC games too?